External Features

The often tube-like body of an insect is composed of a series of segments: six in the head, three in the thorax and up to 11 in the abdomen. Each segment is formed from up to four more or less horny plates called sclerites - a dorsal tergum, a ventral sternum and two lateral pleura (pleura are absent from the insect abdomen). These plates and the various adjacent body segments may be fused together rigidly or joined by soft, flexible membranes that allow for body movement. The body appendages, such as the legs, are formed as outgrowths from the pleura. Where fusion has occurred (particularly in the head) the segments, or their individual components, are not always distinguishable; in the thorax, the sclerites are themselves often subdivided into smaller plates.

The body of an insect is covered by a protective three-layered skin (cuticle) formed from chitin and protein. Depending upon its precise composition and thickness, the cuticle may be soft and flexible or hard and rigid; according to requirements, it may or may not be permeable or waterproof. Following its deposition, the cuticle becomes more or less hardened and darkened by the addition of melanin, during a process called sclerotization. Pre-adult (immature) insects usually moult from one growth stage to the next, sloughing off or bursting out of the 'old' cuticle and replacing it with a larger one; each moult is called an ecdysis. Except in certain very primitive forms, the cuticle of an adult insect is not replaceable. External features of the cuticle (e.g. details of punctation and sculpturing in adult beetles) are often characteristic of the species. Further, the insect body is often adorned with bristles, hairs, scales, setae or spines, and these cuticular outgrowths are also of considerable help in identifying groups or individual species. Immediately beneath the cuticle lies an almost continuous single layer of cells (the epidermis); it is the epidermis that secretes the cuticle. Some epidermal cells are differentiated into glands that may secrete compounds to the outside via ducts that end in characteristic cuticular pores. Glandular secretions include pheromones, silk, wax and other products.

The insect head is essentially a capsule which encloses the brain and bears the usually external (ectognathous) mouthparts, the eyes and a pair of sensory antennae. The mouthparts comprise five basic sections: an 'upper lip' (labrum), the

Fig. 1 General structure of an insect.

Fig. 1 General structure of an insect.

labial palp

Fig. 2 Frontal view of the head of a generalized insect.

labial palp

Fig. 2 Frontal view of the head of a generalized insect.

lower (ventral) surface of which forms the epipharynx; a tongue-like hypopharynx; the jaws (mandibles); the paired maxillae and a 'lower lip' (labium). The mouthparts may include up to two pairs of sensory, feeler-like palps (labial palps and maxillary palps) (Fig. 2). Various pairs of glands (labial glands, mandibular glands, maxillary glands and thoracic glands) are also asso ciated with the mouthparts. The basic biting mouthparts of an insect may be modified considerably for piercing, lapping or sucking (Fig. 3). Piercing mouthparts, as in aphids and other Hemiptera, include a hollow, needle-like stylet (or stylet bundle), formed from the mandibles and the maxillae; the piercing stylet is partly guided by the labium, which forms a long, supportive rostrum (see Fig. 3a); some insects with such mouthparts inject toxic saliva into plants and can cause extensive galling. Many insects have a pair of large, multifaceted compound eyes. Insects may also possess simple eyes (ocelli) which, in adults, may occur as a set of three on the top of the head to form an ocellar triangle. Antennae are present in most adults and in many immature insects; they are, however, often inconspicuous in the latter. The antennae are often slender and feeler-like, but the individual components or subdivisions (the so-called 'segments' or antennomeres - the former term widely used as a term of convenience) are sometimes much elaborated; the number of antennal segments ranges from one to over a

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Fig. 3 Examples of insect mouthparts: (a) piercing mouthparts of an hemipterous bug. including transverse sections through the rostrum (above) and feeding stylet (= stylet bundle) (below): (b) lapping mouthparts of a house fly; (c) sucking mouthparts of a butterfly.

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Fig. 3 Examples of insect mouthparts: (a) piercing mouthparts of an hemipterous bug. including transverse sections through the rostrum (above) and feeding stylet (= stylet bundle) (below): (b) lapping mouthparts of a house fly; (c) sucking mouthparts of a butterfly.

hundred. The basal segment (the scape) is often elongate and separated by an often distinct segment (called the pedicel) from the rest of the antenna, which forms the flagellum (the segments of which are called flagellomeres). Various types of antennae are recognizable (see. for example, Fig. 4).

The muscle-filled insect thorax is composed of three segments: prothorax. mesothorax and metathorax. The prothorax is often very large (as in cockroaches, crickets and many beetles) and the dorsal section (called the pronotum) may be shield-like and cover much or all of the head. In some insects (e.g. flies), the mesothorax forms the bulk of the thorax and the prothorax and metathorax are much reduced. In many insects, the hind part of one of the thoracic segments forms a distinct dorsal scutellum and, sometimes, a postscutellum (see Fig. 112).

Each thoracic segment bears a pair of jointed legs. Each leg has four main components: coxa, femur, tibia and tarsus; there is also a small segment, called the trochanter, lying between the coxa and femur (Fig. 5). In some insects (e.g.

agromyzid flies), the basal part of the tibia may be distinctly coloured and is often called the 'knee". The tarsus is typically multisegmented and terminates in a small pad (the arolium), located between a pair of small tarsal claws. Although insect legs have the same basic structure, they are often considerably modified. The forelegs, for example, may be raptorial (modified for grasping) as in mantises. or fossorial (modified for digging) as in mole crickets; the hindlegs are often saltatory (modified for jumping), as in grasshoppers and flea beetles. Fine details of leg structure (e.g. in beetles, the number of tarsal segments) are often useful for distinguishing between groups of insects.

Adults of most insects also possess one or two pairs of wings: a pair of forewings arising from the mesothorax and a pair of hindwings arising from the metathorax. The base of the wings may be covered by scale-like lobes (the tegulae) or, as in certain flies, protected by membranous folds (the squamae). Wing-coupling arrangements for four-winged insects vary considerably. For example, in some instances (e.g. aphids and

Fig. 5 Segmentation of the leg of an insect, based on the mid-leg of an adult chafer.

bees) a series of hooks (hamuli) on the leading edge (costal margin or costa) of the hindwing interlock with a fold in the trailing edge (dorsal margin or dorsum) of the forewing; in Lepi-

doptera, the wings may be held together in flight by a frenulum (a long bristle or long bristles arising from the hindwing which interlock with a retaining hook - the retinaculum - or a set of small bristles on the underside of the forewing) or by a jugum (a narrow lobe projecting from near the base of the forewing).

The basic arrangement of veins in the wing has undergone considerable modification in the various insect orders, and details of wing venation often form the basis for distinguishing between groups and, sometimes, individual species. The venation of a generalized wing is composed of six main elements: costa (C), sub-costa (Sc), radius (R), media (M), cubitus (Cu) and anal (A). Wings of some insects also include a pigmented patch (pterostigma), usually located near the apex. Some wing veins may be fused or absent, whereas others may be subdivided or distinctly forked. The radius, for example, often branches to produce a curved radial section (Rs) which may itself branch more than once before c Sc Sc c Sc Sc

Fig. 6 Diagram to show the main veins of a generalized insect wing.

reaching the wing margin (Fig. 6). The costal vein and, usually, the anal veins, however, are unbranched. Cross-veins (e.g. m-cu, which links the media with the cubitus) may occur. Areas of the wing membrane delimited by veins are called cells; these may extend to the wing margin (open cells) or may be entirely surrounded by veins (closed cells). Wings have also undergone considerable structural modification. In beetles, cockroaches and earwigs, for example, the forewings are no longer used in flight but have become hardened, leathery flaps known as elytra (beetles and earwigs) or tegmina (cockroaches, etc.); these protect the membranous hindwings which are folded away beneath them when not in use. The forewings (hemelytra) of certain bugs (Heteroptera) are hardened but have a membranous tip. In true flies (Diptera). where only the forewings are used for flying, the hindwings have become reduced to small, drumstick-like balancing organs called halteres.

The many-segmented abdomen is formed from a series of up to 11 dorsal sclerites called tergites and ventral sclerites called sternites, joined by more or less flexible intersegmental membranes. The anterior and posterior segments are often fused or much reduced in size, particularly in adults. The eleventh abdominal segment, for example, is usually very small and inconspicuous; it is totally absent in the higher (most advanced) insects. Some primitive insects (Protura and Collembola) have fewer than ten abdominal segments; Collembola never possess more than six abdominal segments. Abdominal appendages occur on most segments of primitive insects (subclass Apterygota) but are restricted to the hind-most segments of members of the subclass Pterygota. Those of the eighth and ninth segment form the genitalia, including (when present) the female ovipositor and the male claspers. Microscopic features of male and female genitalia are often used by specialists to identify, or to distinguish between, species. Some adult and immature insects possess a pair of cerci, formed from appendages on the last abdominal segment. Cerci are particularly well developed in less-advanced insects (e.g. cockroaches, crickets, earwigs, mayflies) but are usually absent from members of more-advanced groups. Hump-like or suckerlike outgrowths from the ventral body wall of immature insects sometimes form so-called false legs (prolegs or pseudopods); otherwise, ambulatory abdominal appendages, which are commonplace in many arthropods, are wanting in insects.

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